204 Regulation of Size in Unicellular Organisms 



In another species of Spirogyra, Gerassimow ('04a) 

 obtained some cells with nuclear contents of three, 

 four, and five times the normal. These arose by the 

 unequal division of the bivalent cells. In these too it 

 turned out that the lateral surface of the cells was 

 nearly proportional to the nuclear volume, and that 

 cell division was delayed initially only. 



In Spirogyra, therefore, it is clearly evident that cy- 

 toplasmic mass is correlated with nuclear volume. 

 But no factors are so far known which in turn control 

 nuclear or chromatin volume. Gerassimow('04b) made 

 a particular analysis of this question. In some cases 

 very small nuclei were distributed to daughter cells 

 through a process of nuclear fragmentation resulting 

 from treatment with low temperatures or with anes- 

 thetic substances such as chloral. Nuclei smaller than 

 half the normal size never gave rise to long successions 

 of progeny but always died out; while double nuclei 

 continued for many cellular generations. Frequently, 

 however, the nuclear material separated into normal- 

 sized nuclear masses and each mass was isolated into 

 a separate cell. This process of adjustment could not 

 be traced either to nuclear or to cytoplasmic factors. 

 Cells with nuclear masses larger than double the nor- 

 mal did not reproduce their sizes for any large series of 

 generations, but regulated back to smaller numbers of 

 nuclei by multiple fissions. 



What is the meaning of the constancy of the ratio 

 between nuclear volume and cell surface in Spirogyra? 

 In this type of cell the cytoplasm proper is spread in a 

 thin layer over the cell surface. If the thickness of the 

 layer were uniform for all cells, then a double volume 

 of cytoplasm would give a doubling of cell surface. 

 This would mean that the volume of cell sap is care- 

 fully controlled, so as to keep the thickness of the cy- 

 toplasmic layer constant. Or, the surface may con- 



